Area Forecast Discussion
Issued by NWS Des Moines, IA

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431
FXUS63 KDMX 152331
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
531 PM CST Fri Dec 15 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 350 PM CST Fri Dec 15 2017

The main forecast concern was focused on temperatures Saturday
and how warm they reach. Certainly confident to trend warmer with
the current WAA set up today and temperatures not having an issue
warming above guidance. Especially with the cloud cover earlier
today and there looks to be much less or at least mainly high
clouds on Saturday. MOS guidance for MAV and even the ECE have
warmed max temps tomorrow and went at or slightly above across the
entire forecast area.

.LONG TERM.../Saturday night through Friday/
Issued at 350 PM CST Fri Dec 15 2017

Saturday night into Sunday...500mb shortwave and open surface
wave track northeast across the Southern Plains Saturday night and
skirts across Missouri into Illinois Sunday morning. System is
moisture starved but may bring some light precipitation to
southern Iowa Sunday morning from 12z to 18z time frame. Not
overly concerned with precipitation type as surface temperatures
quickly warm above freezing Sunday morning. Plus, there is a
fairly deep dry layer to contend with early in the event and have
low confidence with much QPF out this system. NAM is slightly more
aggressive and faster in completely saturating the column than
the GFS, but models are in decent agreement with ice introduction
and suggest a brief period of snow early Sunday. No snow
accumulation anticipated with temperatures warming and with
precipitation switching over the rain. Low confidence with
mentioning freezing rain attm with surface temperatures and wet
bulbs warming above freezing by 12z Sunday.

Monday through Wednesday...A nice warm and dry period for late
December with the only concern being a weak trough to move through
the region Monday night into Tuesday. This looks to bring some
windy conditions on Tuesday with surface high pressure building
into the Central Plains.

Thursday into Friday...This is the period to watch with the GFS
and ECMWF showing subtle signs of coming into better agreement
with timing and location of trough to impact Iowa Thursday night
into Friday. The ECMWF is much more bullish than the GFS with the
QPF Thursday night, but even still suggest light amounts across
the forecast area. Both models are depicting a long wave trough
across the western CONUS to be split into two separate systems
during the day Friday with a strong cut-off low churning over the
Desert Southwest and the potential surface low and upper level
shortwave to impact Iowa by Thursday night into Friday. The ECMWf
is a bit slower and further south with bringing the QPF across the
area. Certainly suggests a decent deformation zone to set up over
Iowa into southern Minnesota and eastern Nebraska Thursday night.
If the ECMWF 12z solution comes trues, winds behind the system do
look to be fairly stout into Friday and the main question with
this system is will there be any falling snow during that time.

The 12z GFS Bufkit soundings from DSM, OTM, and CSQ do not suggest
saturation within the snow growth region until late Thursday night.
By then, there is a brief period when of saturation in the snow
growth region at the top of the mix layer and lead to the snow
squall potential.  However, the GFS is a bit weaker with the winds
but certainly look strong enough to provide some concern for travel
Thursday night into Friday. There is still several model runs to
hone down this forecast but it is something to keep an eye on due to
the Christmas travel as well as Iowa`s lack of any significant
snowfall so far this season.

&&

.AVIATION.../For the 00Z TAFS through 00Z Saturday evening/
Issued at 529 PM CST Fri Dec 15 2017

VFR conditions forecast through the period with only high level
clouds expected. Light south to southwest winds overnight will
remain light through much of Saturday. Localized LLWS possible
over srn IA overnight including the KDSM/KOTM vicinities.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Podrazik
LONG TERM...Podrazik
AVIATION...Donavon



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